Semiconductor nanostructures can be incorporated into a variety of electronic and optical devices. The electrical and optical properties of such nanostructures vary, e.g., depending on their composition, shape, and size. For example, size-tunable properties of semiconductor nanoparticles are of great interest for applications such as light emitting diodes (LEDs), lasers, and biomedical labeling. Highly luminescent nanostructures are particularly desirable for such applications.
To exploit the full potential of nanostructures in applications such as LEDs and displays, the nanostructures need to simultaneously meet five criteria: narrow and symmetric emission spectra, high photoluminescence (PL) quantum yields (QYs), high optical stability, eco-friendly materials, and low-cost methods for mass production. Most previous studies on highly emissive and color-tunable quantum dots have concentrated on materials containing cadmium, mercury, or lead. Wang, A., et al., Nanoscale 7:2951-2959 (2015). But, there are increasing concerns that toxic materials such as cadmium, mercury, or lead would pose serious threats to human health and the environment and the European Union's Restriction of Hazardous Substances rules ban any consumer electronics containing more than trace amounts of these materials. Therefore, there is a need to produce materials that are free of cadmium, mercury, and lead for the production of LEDs and displays.
There are two main techniques that have been successful used for the synthesis of CdSe nanocrystals: (1) the hot-injection technique and (2) the heating-up technique. Mushonga, P., et al., J. Nanomaterials 2012: Article ID 869284 (2012).
The hot-injection technique involves the rapid injection of a solution of precursors at room temperature into a hot reaction medium in the presence of carefully chosen surfactant molecules. Rapidly injecting the solution of precursors induces a sudden supersaturation and results in a short burst of nucleation. The depletion of reagents through nucleation and the sudden temperature drop associated with the introduction of room temperature reagents results in a decrease in nucleation and an increase in growth of nanocrystals. It is the sequential separation of the nucleation and growth phases that leads to precise control of the size and shape of the resultant nanocrystals. Murray, C. B., et al., J. Am. Chem. Soc. 115:8706-8715 (1993) describes the synthesis of nearly monodisperse CdSe nanocrystals by injection of a room temperature solution combining Me2Cd in trioctylphosphine (TOP) and trioctylphosphine selenide (TOPSe) into a 300° C. solution of tri-n-octylphosphine oxide (TOPO). After the temperature dropped, the reaction mixture was slowly heated to 230-260° C. which allowed for slow growth and annealing of the crystals. Murray at 8708.
The hot-injection technique was also used to synthesize CdTe nanocrystals using a mixture of oleylamine and octadecene as the solvent. Jin, X., et al., J. Nanoparticles 2013: Article ID 243831 (2013). Jin describes preparation of a Te precursor solution which was quickly injected into a Cd precursor solution heated to 300° C. at a weight ratio of Cd to Te of 10:7. Jin found that various shapes of CdTe nanocrystals could be obtained by changing the reaction conditions—e.g., the composition of the solvent.
The heating-up technique is a batch process in which all the precursors are mixed at room temperature followed by a rapid heating of the system to the appropriate growth temperature for the nanocrystals. Mushonga, P., et al., J. Nanomaterials 2012: Article ID 869284 (2012). The heating-up technique allows the supersaturation level and the temperature of the solution to increase together and the nucleation rate is sensitive to both. This technique was found to be easily scaled up and reproducible. Kwon, S. G., et al., J. Am. Chem. Soc. 129:12571-12584 (2007).
In the traditional synthesis of core-shell nanocrystals, cores and ligands are blended in a reactor and heated to the shell growth temperature, and then shell precursors are introduced to the blend to initiate shell growth. During the heat-up, the tiny cores may endure unwanted size, morphology, and/or compositional changes, especially when the desired shell growth temperature is high, in order to encourage reaction of precursors with low reactivity. The issue is more serious when the desired shell growth temperatures are high. To avoid this problem, a temperature ramp-up approach is commonly adopted, in which shell precursors are added while the reaction temperature is ramped up from a low starting temperature to a higher temperature. This approach often results in secondary nucleation issues because at low temperatures, the shell precursors react incompletely and begin to accumulate. Secondary nucleation is nucleation which occurs, irrespective of the mechanism, only because of the presence of crystals of the material being crystallized—no secondary nucleation will occur if no crystals are present. With the temperature increasing and the precursor concentration building up, undesirable secondary nucleation often occurs.
The present invention provides an improved method for the synthesis of nanocrystals. In each increment of the core-precursor mixture introduced into the hot solvent mixture, the core-to-precursor ratio is constant and the precursor concentration is sufficiently low due to the immediate dilution by the solvent mixture. These, along with the high temperature of the reaction mixture, ensure that the shell precursors are consumed rapidly and there is no build-up of precursors before the next increment of core-precursor blend is introduced. Therefore, the opportunity for secondary nucleation is low. The shell precursors will also be equally distributed to each core because of the low precursor concentration, making shell thickness on each core roughly equal. Once a shell is deposited on the cores, the particles become thermodynamically more stable, thus the freshly introduced shell precursors tend to grow on the freshly introduced, thermodynamically more unstable cores. Thus, the cores that arrived earlier do not compete with the newly arrived cores for precursors. This ensures homogeneous shell growth.